pw_intrusive_ptr: Add missing header

Original CL at http://cl/504998828.

Change-Id: I509634aec51286e93efd28e8814e01fd7a51c523
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/126990
Pigweed-Auto-Submit: Rob Mohr <mohrr@google.com>
Pigweed-Auto-Submit: Alexander Kornienko <alexfh@google.com>
Commit-Queue: Auto-Submit <auto-submit@pigweed.google.com.iam.gserviceaccount.com>
Reviewed-by: Alexander Kornienko <alexfh@google.com>
Reviewed-by: Anthony DiGirolamo <tonymd@google.com>
diff --git a/pw_intrusive_ptr/public/pw_intrusive_ptr/intrusive_ptr.h b/pw_intrusive_ptr/public/pw_intrusive_ptr/intrusive_ptr.h
index b525328..0b86230 100644
--- a/pw_intrusive_ptr/public/pw_intrusive_ptr/intrusive_ptr.h
+++ b/pw_intrusive_ptr/public/pw_intrusive_ptr/intrusive_ptr.h
@@ -14,6 +14,7 @@
 #pragma once
 
 #include <cstddef>
+#include <type_traits>
 #include <utility>
 
 #include "pw_intrusive_ptr/internal/ref_counted_base.h"